1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, and particularly relates to non-reciprocal circuit elements such as isolators, circulators, and the like, preferably for use in microwave bands, for example.
2. Description of the Related Art
Non-reciprocal circuit elements such as isolators, circulators, and the like have conventionally had characteristics in which signals are transmitted only in a predetermined specific direction, and are not transmitted in the reverse direction. Using such characteristics, an isolator, for example, is used as a sending circuit portion in a mobile communication device such as a cellular phone or the like.
As this type of non-reciprocal circuit element, WO 2008/087782 discloses a two-port isolator configured of a permanent magnet, a ferrite to which a direct current magnetic field is applied by the permanent magnet, a first center electrode and a second center electrode disposed so as to intersect around the ferrite and be electrically insulated from each other, and so on.
According to this isolator, in the case where the first and second center electrodes are designed so as to have a desired inductance value, the number of times the first and second center electrodes are wrapped around the ferrite, the width of the lines, the shape (depth) of through-holes used for connections, and the thickness of the ferrite serve as parameters. However, reducing the size of the isolator also makes it necessary to reduce the size of the ferrite, which in turn reduces the freedom with which the various parameters can be set, making it difficult to achieve a desired inductance.
Inductance of the center electrodes can be reduced by reducing the number of times the electrodes are wrapped, increasing the line width, increasing the depth of the through holes, and reducing the thickness of the ferrite. However, reducing the number of times the electrodes are wrapped will cause a drastic drop in the inductance. In other words, inductance is proportional to the square of the number of times the electrodes are wrapped, and thus if three turns are reduced to two terms, for example, the inductance will drop by approximately 55%. The coupling of the first and second center electrodes will also drop, causing a degradation in the insertion loss characteristics.
Meanwhile, increasing the line width of the center electrodes or increasing the depth of the through holes results in an increased chance of shorts between the electrodes, which is problematic in terms of reliability. Furthermore, reducing the thickness of the ferrite is problematic because doing so also reduces the strength of the ferrite, which in turn increases the chance of breaks, cracks, and so on being produced in a ferrite substrate during a polishing process, a process for forming the center electrodes, and so on.
In recent years, the operational frequencies of non-reciprocal circuit elements are moving toward high-frequency bands, of 2 GHz or greater. In such a case, the optimal inductance for the center electrodes is low, and thus the aforementioned problems appear prominently. Furthermore, inter-line stray capacitance is generated unavoidably between the center electrodes, resulting in the center electrodes self-resonating at a constant frequency, and thus a non-reciprocal circuit element that operates at a frequency band greater than or equal to the self-resonating frequency cannot be realized. As such, it is necessary to reduce the inductance of the center electrodes and increase the self-resonating frequency in order to realize a non-reciprocal circuit element having a high operational frequency.